You are working in a microbiology laboratory culturing bacteria that require an acidic environment and you need to prepare a buffer to keep the culture at an appropriate pH. You prepare a buffer solution that is 0.040 m NaCH₃CO₂(aq) and 0.080 m CH₃COOH(aq) at 25°C. What is the pH of the solution?

ANTICIPATE If the weak acid were present alone, you would expect pH < 7. Because some conjugate base has been added, you should expect the pH to be slightly higher than the acid-only value (which is estimated to be about 2.92 in Example 6D.1) but still less than 7.

PLAN First, identify the weak acid and its conjugate base. Then, write the proton transfer equilibrium between them and rearrange the expression for Ka to give [H₃O⁺]. Finally, calculate the pH.

What should you assume? Assume that the protonation of acetate ions and the deprotonation of acetic acid molecules have both proceeded to such a small extent that the concentrations of both species are nearly the same as their initial (formal) values.